U.S. patent number 4,081,472 [Application Number 05/711,823] was granted by the patent office on 1978-03-28 for process for preparation of aromatic isocyanates.
This patent grant is currently assigned to Mitsui Toatsu Chemicals Inc.. Invention is credited to Takeshi Abe, Usaji Takaki, Ryuichirou Tsumura.
United States Patent |
4,081,472 |
Tsumura , et al. |
March 28, 1978 |
**Please see images for:
( Certificate of Correction ) ** |
Process for preparation of aromatic isocyanates
Abstract
A process is described for preparing aromatic isocyanates by
contacting an aromatic carbamate of the formula R(NHCO.sub.2 R')n
at 150.degree.-350.degree. C under reduced pressure with one or
more metal or metal compound catalysts dissolved in an inert
solvent. In the formula R is a divalent aromatic, R' is a
monovalent aliphatic, alicyclic or aromatic with at most 8 carbon
atoms and n is 1 or 2. The metal is dissolved in a concentration of
at least 1/1000% by weight of the solvent. The vapors of the
resulting aromatic isocyanate and alcohol or phenol are
fractionally condensed under reduced pressure and the aromatic
isocyanate is separately collected.
Inventors: |
Tsumura; Ryuichirou (Yokohama,
JA), Takaki; Usaji (Kamakura, JA), Abe;
Takeshi (Tokyo, JA) |
Assignee: |
Mitsui Toatsu Chemicals Inc.
(Tokyo, JA)
|
Family
ID: |
14154777 |
Appl.
No.: |
05/711,823 |
Filed: |
August 5, 1976 |
Foreign Application Priority Data
|
|
|
|
|
Aug 7, 1975 [JA] |
|
|
50-96056 |
|
Current U.S.
Class: |
560/345;
560/352 |
Current CPC
Class: |
C07C
263/04 (20130101); C07C 263/04 (20130101); C07C
265/14 (20130101); C07C 263/04 (20130101); C07C
265/12 (20130101) |
Current International
Class: |
C07C
265/14 (20060101); C07C 265/12 (20060101); B01J
31/02 (20060101); B01J 31/16 (20060101); C07C
263/00 (20060101); C07C 265/00 (20060101); C07C
263/04 (20060101); C08G 18/76 (20060101); C08G
18/00 (20060101); C07C 118/00 () |
Field of
Search: |
;260/453P |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Torrence; Dolph H.
Claims
What is claimed is:
1. A process for the preparation of aromatic isocyanates,
comprising the steps of: providing an aromatic carbamate of the
general formula:
wherein R is selected from monovalent and divalent aromatic groups
with at most 32 carbon atoms, R' is selected from monovalent
aliphatic, alicyclic and aromatic groups with at most 8 carbon
atoms, and n is an integer selected from 1 and 2; contacting said
aromatic carbamate at a temperature of 150.degree.-350.degree. C
and subatmospheric pressure with an solution of at least one
compound selected from the group consisting of, carboxylates;
alcoholates; phenolates; sulfonates; chelates with a chelating
agent selected from .beta.-diketones, ketoesters, hydroxyaldehydes,
amino acids and hydroxyacids; carbamates; thio-and
dithiocarbamates; hydroxides; nitrates; phosphates; borates;
complexes with a neutral ligand selected from amines, phosphines,
phosphites, nitriles and amides; and oxides of at least one metal
selected from the group consisting of copper, zinc, aluminum, tin,
titanium, vanadium, iron, cobalt and nickel as catalyst, said
catalyst being dissolved in an inert solvent having a boiling point
of at least 200.degree. C in a metal concentration of at least
0.001% by weight based on said solvent, to effect the pyrolysis of
said aromatic carbamate and to obtain vapors of aromatic isocyanate
and by-product vapors; thereafter subjecting all of said vapors to
fractional condensation under subatmospheric pressure to collect
said aromatic isocyanate separately from said by-product
vapors.
2. The process according to claim 1 wherein the reaction pressure
is at most 400 mmHg absolute and the reaction temperature is
200.degree.-300.degree. C.
3. The process according to claim 2 wherein said inert solvent is a
high boiling petroleum fraction selected from the paraffin series,
the naphthene series and the aromatic series.
4. The process according to claim 1 wherein said aromatic carbamate
is an alkyl ester selected from alkyl esters of
tolylene-2,4-dicarbamic acid and alkyl esters of
tolylene-2,6-dicarbamic acid.
5. The process according to claim 4 wherein a
monoisocyanatomonocarbamate is formed as an intermediate product of
reaction and is recycled to the reactor.
6. The process according to claim 2 wherein the metal part of said
catalyst is at least one metal selected from zinc, aluminum, tin,
and titanium.
7. The process according to claim 4 wherein the metal part of said
catalyst is aluminum.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a process for the preparation of
aromatic isocyanates wherein aromatic carbamates (referred to
hereinafter as the carbamates) are pyrolyzed by bringing them into
contact with a catalyst dissolved in a solvent inert to
isocyanates.
Isocyanates are very useful substances chiefly as starting
materials for polyurethanes. In particular, tolylene diisocyanate,
methylene-bis-(4-phenyl isocyanate) and naphthylene diisocyanates
are now prepared on a large commercial scale.
A current process for preparing these isocyanates, for example,
tolylene diisocyanate of the formula: ##STR1## comprises nitrating
toluene to form dinitrotoluene, reducing the latter with hydrogen
to form the corresponding diamine and then reacting the diamine
with phosgene. Namely, the current process comprises complicated
and troublesome steps, requring the use of a large amount of highly
toxic phosgene and permitting the formation of hydrogen chloride as
by-product. In this field, therefore, there is a great demand and
research for a new and improved process which requires no phosgene
and is simpler and more economical than the above process.
One process which might be considered as a substitute for the
current process employing phosgene is a process for preparing
isocyanates wherein carbamates are pyrolyzed. Such a process, which
relatively easily synthecizes carbamates directly from mitro
compounds was developed in recent years. However, the known
conventional pyrolysis process is industrially inoperable or
economically disadvantageous in respect of yield of isocyanates,
reaction rate, materials of construction, control of temperatures
and elimination of by-products, and is consequently not practiced
as a suitable process for preparing isocyanates.
It has now been found that the above mentioned drawbacks can be
overcome when carbamates are subjected to a liquid phase pyrolysis
under reduced pressure in the presence of specific catalysts.
The reaction for forming isocyanates by pyrolysis of carbamates may
be shown by the following basic equation:
On thermal dissociation of the carbamate, several undesirable side
reactions take place at the same time. These side reactions are:
the decarboxylation reaction of the carbamate accompanying the
formation of a primary amine RNH.sub.2 and an olefin or of a
secondary amine RNHR' as by-product; the reaction between the
produced isocyanate and the starting carbamate, permitting the
formation of an allophanate as by-product; the reaction between the
produced isocyanate and an amine formed as by-product permitting
the formation of a urea compound as by-product; and the
polymerization of the produced isocyanate, permitting the formation
of an isocyanurate or a polymer as by product. The thermal
dissociation reaction of equation (1) above is reversible and its
equilibrium remains with the left-hand side carbamate at normal
temperature but is shifted to the right-hand side by heating,
whereby the dissociation of the carbamate takes place. In this
case, the thermal dissociation temperature varies according to the
sort of carbamate and the reaction conditions. Accordingly, it is
important for obtaining isocyanates advantageously from carbamates
to perform the pyrolysis reaction of equation (1) selectively while
inhibiting the above mentioned side and reverse reactions.
The conventional pyrolysis of carbamates is roughly classified into
reactions carried out in the vapor phase at a high temperature and
reactions carried out in the liquid phase at a relatively low
temperature. U.S. Pat. No. 3,734,941 discloses a typical vapor
phase process wherein a carbamate is pyrolyzed at
400.degree.-600.degree. C in the presence of a Lewis acid and the
resultant vapor is separated by fractional condensation into an
isocyanate and an alcohol. According to this process, for example,
tolylene diisocyanate is obtained in a yield of 60% by pyrolysis of
diethyl tolylene-2,4-dicarbamate of the formula: ##STR2## in the
presence of ferric chloride. However, this process has the
drawbacks of a low yield of the product, decomposition of the
catalyst, corrosion of the reaction apparatus at high temperatures,
and formation of a considerable amount of a polymer as by-product.
German Pat. No. 2,410,505 proposed as an improved vapor phase
method a process wherein the residence time of the reactants at
350.degree.-550.degree. C is controlled within 15 seconds.
According to this improved process, the yield of isocyanate as high
as 93%, although the carbamate has to be supplied in the form of
powders to the reaction zone. However, a solid polymer is also
formed by this improved process as by-product and is gradually
deposited in the reactor and in the condenser during the course of
sustained operation, thus making it difficult to conduct a
continuous reaction. In addition, a large quantity of heat required
for the endothermic pyrolytic reaction has to be supplied to the
starting material within a very short period of time. This
additional factor causes this improved process to encounter great
difficulty in being adopted into practice.
If a liquid phase pyrolysis of the carbamates could be performed at
a high reaction rate to afford the end product in a high yield and
at a temperature lower than that adopted in the vapor phase
methods, elimination of by-products and supply and control of the
heat of reaction would become easier and the process as a whole
would become very advantageous. According to U.S. Pat. No.
2,409,712, a reaction mixture containing ethoxyethoxyethyl
N-laurylcarbamate, synthesized in a yield of 57% by heating
laurylamine, urea and ethoxyethoxyethanol at 200.degree. C for 3
hours, is subjected directly to liquid phase pyrolysis conducted at
210.degree.-230.degree. C under reduced pressure of 2 mmHg whereby
lauryl isocyanate is isolated in a yield of 75%. This fact shows
that a liquid phase pyrolysis of carbamates to isocyanates takes
place relatively easily. However, such yield is still too low to be
practical. This is due to the reason that in the case of liquid
phase pyrolysis of carbamates in the absence of a solvent or in the
presence of a solvent containing even such a substance (reactive
with isocyanates) as mentioned above, the concentration of the
reactants, i.e. the concentration of functional groups such as OH,
NCO and NHCO.sub.2 R becomes extremely high and the reaction time
also increases when compared with the above mentioned vapor phase
pyrolysis, so that the various above mentioned side reactions tend
to take place and this tendency is more noticeable than the
inhibition of the side reactions by lowering the temperature. It is
known already to promote thermal dissociation while inhibiting side
reactions by diluting the reactants with an inert solvent to lower
the concentration of the functional groups.
It has been reported that, as a result of performing the pyrolysis
of carbamates in the presence of various amines or fatty acids in
an inert solvent such as a hydrocarbon, ether or nitrobenzene, the
rate of thermal dissociation is increased as the acidity of
alkalinity becomes stronger or as the polarity of the inert solvent
becomes higher. [Mukaiyama et al., J. Amer. Chem. Soc. 78, 1946),
Bull. Chem. Soc., Japan 33, 1137 (1960)]. It has also been reported
that, as result of measuring the thermal dissociation temperatures
of various carbamates during the thermal dissociation reaction of
various carbamates to tolylene diisocyanates in an inert solvent
selected from a paraffin oil and methoxypolyethylene glycol, the
thermal dissociation temperature is lower in the case of using the
latter inert solvent [G. R. Griffin et al., I & EC Product
Research and Development 1, 265 (1962)]. It is thus evident from
these reports that a thermally stable solvent with high polarity is
desirable as an inert solvent for pryolysis of carbamates.
German patent publication DOS No. 2,421,503 discloses a method of
separately recovering isocyanates and alcohols by dissolving
carbamates in an inert solvent such as hydrocarbon, ether, ketone
or ester and pyrolyzing the carbamates at 175.degree.-350.degree. C
under atmospheric or superatmospheric pressure in the presence of a
carrier. Carbamates shown in 20 examples of this publication are
those capable of forming phenyl isocyanate, tolylene diisocyanate
and hexamethylene diisocyanate. The yield of the isocyanate
products obtained by separating alcohols formed by thermal
dissociation of the carbamates is 22-84% in all examples. The
synthesis rate of the isocyanate products is 23-84 g/liter per hour
in 6 examples wherein the yields of the isocyanate products are at
least 70%. The best yield is obtained in an example wherein
tolylene diisocyanate is obtained by pyrolysis of
tolylene-2,4-dicarbamate. In this example, the reaction is carried
out by continuously supplying n-hexadecane and a solution of the
dicarbamate in tetrahydrofuran into a flask charged with
n-hexadecane as inert solvent, pyrolyzing the dicarbamate at
250.degree. C under slightly superatmospheric pressure while
blowing a large amount of nitrogen as carrier into the flask and
recovering the pyrolyzed fraction distilled at 180.degree. C over
the top of a fractionating column fitted to the flask after an
average resident time of about 20 hours. In a steady stage, the end
product, i.e. tolylene diisocyanate is recovered at a yield of 84%
and a monocarbamate at a yield of 9%. In this case the synthesis
rate of the diisocyanate is about 21 g/liter per hour. The yield
obtained in this method is higher than that of the aforementioned
U.S. Pat. No. 2,409,712 but is still too low to be practical. In
addition, a problem arises in that the reactants are diluted with a
solvent so as to decrease the synthesis rate of the isocyanate (the
yield of the isocyanate aimed at per unit volume per hour).
Accordingly, the isocyanates and the carbamates which easily
undergo the above mentioned various side reactions have to be
maintained at a high temperature for a long period of time, thus
causing a decrease in the yield of the end product. Further, a
considerably large capacity is required, thus making the process
economically unacceptable. In this liquid phase process, therefore,
the yield of the product is not satisfactory and the reaction rate
is too small.
The above reference discloses that an acid such as a fatty acid or
sulfuric acid or a base such as an amine functions as a catalyst
for the thermal dissociation reaction in the liquid phase of
carbamates to isocyanates and that the reaction is promoted more
smoothly as the acid or base becomes stronger. As such acid or base
reacts with the resultant isocyanates, however, the acid or base
can no longer be used as catalyst.
Recently a process for preparing isocyanates has been described in
U.S. Pat. No. 3,919,278 wherein a mononuclear aromatic carbamate is
dissolved in an inert solvent in an amount such that the total
concentration of the carbamate and a product obtained by pyrolysis
thereof is within a range of about 1-20 mol % and the pyrolysis of
the carbamate is carried out at 230-290.degree. C in the presence
of an inert carrier used in an amount of at least 3 molar
proportion to the carbamate. Another process for preparing
isocyanates is described in U.S. Pat. No. 3,919,279 wherein a
carbamate is dissolved in an inert solvent and brought into contact
at a high temperature with a catalyst composed of a heavy metal
(Mo, V, Mn. Fe, Co, Cr, Cu or Ni) or a compound thereof to effect
the pyrolysis of the carbamate. The former process (U.S. Pat. No.
3,919,278) is almost identical to that disclosed in the above
mentioned German DOS No. 2,421,503, and requires the use of a large
amount of a carrier and can hardly be carried out under
subatmospheric pressure. The latter process (U.S. Pat. No.
3,919,279) although it uses as a catalyst a specific metal or a
compound thereof, is limited to using such catalyst under the
reaction conditions in the presence of a carrier under atmospheric
or superatmospheric pressure and so cannot be carried out under
subatmospheric pressure.
DESCRIPTION OF THE INVENTION
It has now been found surprisingly that although various kinds of
metal compounds exhibit a promoting action to the thermal
dissociation reaction of carbamates including side reactions, only
specific metals, i.e. those belonging to Groups I-B, II-B, III-A,
IV-A, IV-B, V-B and VIII of the Periodic Table strongly function
catalytically, especially in the dissolved state and their
catalytic action does not essentially depend on the form or
combination of the starting materials for the catalyst and the
combining ingredients.
Based on the above finding, the present inventors have succeeded in
solving the problems raised by the liquid phase thermal
dissociation reaction of carbamates and have developed a process
for preparing isocyanates in highly acceptable yields and at high
reaction rates.
According to the process of the present invention and contrary to
all prior art, a catalytic pyrolysis reaction can effectively be
carried out under subatmospheric pressure in the absence of a
carrier without dissolving a carbamate necessarily in an inert
solvent. This is a fundamentally different mode of reaction from
the above processes and exhibits, as will be illustrated in
examples given hereinafter, remarkably high reaction rate,
superiority in the cost of starting materials resulting from the
use of no carrier and higher yields.
The prime objects of the present invention, therefore, provide the
following:
1. Process for the preparation of aromatic isocyanates in a very
high yield from the corresponding aromatic carbamates wherein the
reaction is carried out under reduced pressure in the presence of
specific catalysts.
2. A process for the preparation of aromatic isocyanates in a very
high yield wherein aromatic carbamates are pyrolyzed at an
extremely high reaction rate under reduced pressure in the presence
of specific catalysts.
3. A process for the preparation of aromatic isocyanates of high
purity wherein aromatic carbamates are pyrolyzed.
4. A non-corrosive, economical, practical catalyst of long life and
having high catalytic activity and selectivity which is useful in
the process for the preparation of the aromatic isocyanates wherein
aromatic carbamates are subjected to a liquid phase pyrolysis as
well as the use of the catalyst.
5. An inert solvent which is stable to heat and to aromatic
isocyanates, is capable of dissolving the catalyst and discretely
the aromatic carbamates and is useful in the process for the
production of the aromatic isocyanates wherein the aromatic
carbamates are subjected to a catalytic liquid phase pyrolysis
reaction as well as the use of the inert solvent.
6. A process for the preparation of aromatic isocyanates wherein
the supply of the heat of the dissociating endothermic reaction of
aromatic carbamates and control of the reaction temperature are
rendered simple.
7. A process for the preparation of aromatic isocyanates wherein
the operation can continuously be carried out while easily
separating a small amount of by-products with boiling points higher
than those of the aromatic isocyanate.
8. A process for continuously preparing aromatic diisocyanates in a
high yield wherein aromatic dicarbamates are pyrolyzed.
Briefly stated, the process of the present invention is carried out
by bringing an aromatic carbamate into contact at a temperature of
150.degree.-350.degree. C under reduced pressure with a catalyst
dissolved in an inert solvent to effect pyrolysis of the aromatic
carbamate, collecting the resultant isocyanate and alcohol in the
form of vapors, and thereafter separately condensing both vapor
products. In case a diisocyanate is desired as the end product of
the process, an isocyanato-carbamate compound in which only one of
the dicarbamate groups has been dissociated is produced
intermediately. Such intermediate product having a high boiling
point is converted into the desired polyisocyanate by immediately
fractionating the reaction products and refluxing only the high
boiling component back to the reactor or by distilling the desired
polyisocyanate from the isocyanate fraction containing all the
condensed products other than alcohol components and recycling the
still residue to the reactor.
The aromatic carbamates used in the present invention as starting
material are represented by the general formula: R(NHCO.sub.2 R')n
In the formula, R is a monovalent or divalent aromatic group with
at most 32 carbon atoms. R may contain an isocyanato group and a
monovalent or divalent substituent not reactive therewith, R' is a
monovalent aliphatic, alicyclic or aromatic hydrocarbon group with
at most 8 carbon atoms and may contain an isocyanato group or a
monovalent substituent not reactive therewith. And n is an integer
of 1 or 2 and corresponds to the valency of the substituent R.
Illustrative of the substituent R are aryl groups such as phenyl,
tolyl, xylyl, naphthyl, biphenylyl, anthryl, phenanthryl,
terphenyl, naphthacenyl and pentacenyl groups and divalent groups
formed by removing one hydrogen atom from these aromatic groups.
These aromatic groups may contain an isocyanato group; a
substituent not reactive therewith such as an alkyl group, a
halogen atom, nitro group, cyano group, an alkoxy group, an acyl
group, an acyloxy group or an acylamido group; or a divalent
substituent of similar nature such as a methylene group, an ether
group, a thioether group, a carbonyl group or a carboxyl group.
Examples of the substituent R' include aliphatic groups such as
methyl, ethyl, propyl, butyl, hexyl, octyl and methoxyethyl groups;
alicyclic groups such as a cyclohexyl group; and aromatic groups
such as phenyl and tolyl groups.
Typical examples of the carbamates utilizable in the present
invention include methyl phenylcarbamate, ethyl phenylcarbamate,
propyl phenylcarbamate, butyl phenylcarbamate, octyl
phenylcarbamate, ethyl naphtyl-1-carbamate, ethyl
anthryl-1-carbamate, ethyl anthryl-9-carbamate, diethyl
anthrylene-9,10 dicarbamate, ethyl p-biphenylylcarbamate, diethyl
m-phenylenedicarbamate, diethyl naphthylene-1,5-dicarbamate, methyl
p-tolylcarbamate, ethyl p-trifluoromethylphenylcarbamate, isopropyl
m-chlorophenylcarbamate, ethyl 2-methyl-5-nitrophenylcarbamate,
ethyl 4-methyl-3-nitrophenylcarbamate, ethyl
4-methyl-3-isocyanatophenylcarbamate,
methylene-bis-(phenyl-4-ethylcarbamate), dimethyl
tolylene-2,4-dicarbamate, diethyl tolylene-2,4-dicarbamate, diethyl
tolylene-2,6-dicarbamate, diisopropyl tolylene-2,4-dicarbamate,
dibutyl tolylene-2,4-dicarbamate, diphenyl
tolylene-2,4-dicarbamate, diphenyl tolylene-2,6-dicarbamate,
di(ethoxyethyl) tolylene-2,4-dicarbamate, diethyl
4-chlorophenylene-1,3-dicarbamate, methyl p-butoxyphenylcarbamate,
ethyl p-acetylphenylcarbamate, ethyl o-nitrophenylcarbamate and
isopropyl m-trifluoromethylphenylcarbamate. Of these carbamates
compounds, the most practical examples are the
tolylenedicarbamates, naphthylenedicarbamates and
methylene-bis-(phenylcarbamate).
In the present invention, the pyrolysis reaction is carried out by
bringing the carbamate into contact at a temperature of
150.degree.-350.degree. C under reduced pressure with one or more
compounds of metals belonging to Groups I-B, II-B, III-A, IV-A,
IV-B, V-B and VIII of the Periodic Table. These metal compounds
should be dissolved in an inert solvent to have a metal
concentration of at least 0.001% by weight at the reaction
temperature, any insoluble moiety of the metal compounds being
allowed to co-exist. The main catalytic action to pyrolysis of the
carbamates is displayed by the dissolved metal component itself and
not by the combined form thereof or of its combined components.
Accordingly, no particular limitation exists in the combined
components for the metal utilizable as the catalyst and in the
method for using the catalyst. Any method capable of dissolving the
metal component in the inert solvent so as to have the desired
concentration at the reaction temperature may be employed.
Metals belonging to Groups I-B, II-B, III-A, IV-A, IV-B, V-B and
VIII of the Periodic Table or organic or inorganic compounds
thereof may be used as the catalyst utilizable in the present
invention. Preferably, compounds of copper, zinc, aluminum, tin,
titanium, vanadium, iron, cobalt and nickel are employed. Examples
of the metal compounds used as catalyst include metal salts with
aliphatic, alicyclic and aromatic carboxylic acids such as formic
acid, acetic acid, lauric acid, stearic acid, oxalic acid, azelaic
acid, naphthenic acid, tetrahydrophthalic acid, benzoic acid,
phthalic acid and pyromellitic acid; metal alcoholates with
aliphatic and alicyclic alcohols such as methanol, ethanol,
propanol, butanol, octanol, dodecyl alcohol, benzyl alcohol,
ethylene glycol, propylene glycol, polyethylene glycol, glycerol,
pentaerythritol and cyclohexyl alcohol as well as the corresponding
metal thioalcoholates; metal phenolates with monohydric or
polyhydric phenol derivatives such as phenol, cresol, nonylphenol,
catechol and hydroquinone as well as the corresponding metal
thiophenolates; metal salts with sulfonic acids such as
methanesulfonic acid, ethanesulfonic acid, dodecanesulfonic acid,
cyclohexanesulfonic acid, benzenesulfonic acid, toluenesulfonic
acid and dodecylbenzenesulfonic acid; metal chelates with chelating
agents, for example, .beta.-diketones such as acetylacetone and
benzoylacetone, ketoesters such as ethyl acetoacetate and ethyl
benzoacetate, hydroxyaldehydes such as salicylaldehyde and
2-hydroxy-1-naphthaldehyde, amino acids such as glycin, alanine,
aspartic acid, glutamic acid, serine, tyrosine and iminodiacetic
acid, and hydroxy acids such as glycolic acid, lactic acid and
salicylic acid; metal carbamates with carbamates defined as the
starting material for the present invention as well as the
corresponding metal thiocarbamates and dithiocarbamates; metal
salts with compounds having anionic ligands such as hydroxyl group,
nitric acid group, phosphoric acid group, boric acid group and
cyanato group; and metal complexes of the above mentioned various
metal salts with ligands having a non-covalent electron pair such
as amines, phosphines, phosphites, nitriles and amides. These metal
compounds are used as such to catalize the reaction. In case a
metal salt derived from the anionic ligand has a poor solubility in
an inert solvent, such metal salt may be added to the reaction
system exceptionally together with a carboxylic acid, an alcohol, a
phenol, a sulfonic acid or a chelating agent so as to increase the
concentration of the metal in the inert solvent to a given value.
In the case of metal powders or metal oxides, these may be
dissolved in the inert solvent in a manner similar to the
aforementioned.
Preferred examples of catalysts include copper naphthenate, zinc
naphthenate, zinc acetate, zinc oxalate, zinc benzoate, zinc
hexylate, zinc dithiocatecholate, zinc dodecylbenzenesulfonate,
zinc acetylacetonate, zinc N,N-ethylphenyldithiocarbamate, zinc
hydroxide, zinc oxide and naphthenic acid, aluminum benzoate,
aluminum isobutylate, aluminum salicylaldehydate, tin acetate, tin
octanoate, dibutyl tin dilaurate, titanium oxalate, titanium
naphthenate, titanium phenolate, vanadium naphthenate, vanadium
acetylacetonate, iron naphthenate, iron acetoacetate, cobalt
naphthenate, bis-triphenylphosphine cobalt nitrate, nickel
naphthenate and bis-pyridine nickel nitrate.
The preferred amount of the metal compound used as catalyst may be
within a range of 0.01-10% by weight based on the inert solvent
used for the reaction. A most preferred amount of the metal
compound varies according to the sort of catalyst and the mode of
its use, but is usually within a range of 0.05-1% by weight.
The inert solvent utilizable for the reaction of the present
invention is a high boiling inert solvent which has a boiling point
of at least 200.degree. C under normal pressure and is not reactive
with the isocyanate but capable of dissolving the metal compound
used as the catalyst. For example, the inert solvent may be
selected from hydrocarbons, ethers, thioethers, ketones,
thioketones, sulfones, esters, organosilane compounds and mixtures
of these compounds. The solvent should dissolve the metal compound
so as to afford a metal concentration of at least 0.001% by weight
at the reaction temperature. The effect achieved by the use of the
solvent is that the dissolved metal component of the catalyst is
brought into effective and even contact in the liquid or at the
surface with the carbamate in dissolved, suspended or emulsified
dilute state whereby a selective reaction is promptly initiated and
promoted. Thus, this solvent need not necessarily dissolve the
carbamate but desirably may dissolve it. The solvent also functions
as a heat medium serving to supply heat to the reaction system and
to make the reaction temperature uniform. Furthermore, the solvent
serves to remove from the reactor a small amount of by-products
having boiling points higher than the boiling point of the
isocyanate. Useful solvents have a boiling point at least
50.degree. C higher than the boiling point of the isocyanate, are
stable at the reaction temperature, are capable of dissolving both
catalyst and carbamates and are easily available at a low cost.
Preferred examples of hydrocarbons as the inert solvent include
aliphatic hydrocarbons such as the higher alkanes, dodecane,
hexadecane, octadecane, and liquid paraffin, the corresponding
alkenes, petroleum fractions of paraffin series such as those
usually employed as lubricating oils or cutting oils; alicyclic
hydrocarbons such as petroleum fractions of the naphthene series;
aromatic hydrocarbons such as dodecylbenzene, dibutylbenzene,
methylnaphthalene, phenylnaphthalene, benzylnaphthalene, biphenyl,
diphenylmethane, terphenyl and aromatic petroleum fractions usually
employed as rubber-treating oils; and substituted aromatic
compounds having no reactivity with the isocyanate such as
chloronaphthalene, nitrobiphenyl and cyanonaphthalene. Other
preferred inert solvents are, for example, ethers and thioethers
such as diphenyl ether, methyl naphthyl ether, diphenyl thioether
and the like aromatic ethers and thioethers; ketones and
thioketones such as benzophenone, phenyl tolyl ketone, phenyl
benzyl ketone, phenyl naphthyl ketone and the like aromatic ketones
or thioketones; sulfones such as diphenyl sulfone and the like
aromatic sulfones; esters such as animal and vegetable oils,
dibutyl phthalate, dioctyl phthalate, phenyl benzoate and the like
aliphatic and aromatic esters; organosilane compounds such as
conventional silicone oils. Among these solvents, high boiling
petroleum fractions of paraffin series, naphthene series or
aromatic series are especially practical.
Although no limitation exists in the amount of the solvent used,
the amount by weight is usually within a range of 0.1-100 times the
amount of the carbamate, the optimal value depending on the mode of
reaction. In a continuous reaction, the solvent in an amount by
weight of 0.1-10 times the supplied amount of the starting
carbamate is recycled to the reactor. Although a small amount of
by-products having boiling points higher than those of the desired
isocyanate and the alcohol are produced in the course of pyrolysis
of the carbamate, a continuous pyrolytic reaction can be carried
out over a long period of time with a minimum amount of the solvent
without any trouble by withdrawing the by-products from the reactor
together with the solvent, separating the by-products from the
solvent and recycling the solvent alone to the reactor.
In the process of the present invention, the reaction temperature
is within a range from 150.degree. to 350.degree. C. The reaction
temperature should be above the temperature at which dissociation
of the carbamate usually begins (about 150.degree. C). However,
above 350.degree. C, the reaction proceeds too rapidly thus giving
rise to side reactions very pronouncedly and the reaction becomes
less economical in view of the increase in heating cost. An optimum
temperature has to be determined according to the sort of starting
carbamate used, but a temperature within a range from 200.degree.
to 300.degree. C is generally desirable. Although it is necessary
to supply an amount of heat to satisfy the needs of the endothermic
reaction and of the latent heat of evaporation of the products,
this can easily be supplied by the solvent itself, which also
functions as a heat transfer medium.
The reaction is carried out under reduced pressure. A proper
pressure is selected in connection with the temperature according
to the sort of the carbamate used, so as to meet the requirement
for recovering the product as vapor. Usually, an absolute pressure
within a range of 10-400 mmHg is desirable. In special reactions,
however, wherein an isocyanate with a high boiling point is to be
obtained or the thermal degradation of the product is to be
prevented, the reaction may be carried out under an absolute
pressure within a range of 1-10 mmHg in a reactor such as a film
evaporator.
The reaction time is within a range from a few seconds to several
hours. The reaction time varies chiefly according to the sort of
carbamate, the type and amount of catalyst, the reaction
temperature, the reaction pressure and the mode of the reaction
adopted, but it can significantly be shortened by the catalytic
effect achieved by the present invention as compared with the case
of using no catalyst. In Examples 3-6, for instance, the rates of
the formation of isocyanates are increased by about 3 times by the
use of a catalyst.
The process of the present invention may be carried out also
batchwise, but is desirably carried out continuously in a
completely mixing type or an extrusion flow type reactor. In a
continuous process, for example, the carbamate in a powdery or
molten form or as a mixture with the inert solvent is supplied to a
reactor which has previously been charged with a given catalyst and
the inert solvent and has optionally been preheated to a given
temperature under a given pressure. The isocyanate and the alcohol
produced by the pyrolytic reaction are separately condensed by
taking advantage of the difference in their boiling points.
The present invention affords a very high yield of the product
without the aid of any carrier, contrary to the processes of the
above mentioned German DOS No. 2,421,503 and U.S. Pat. Nos.
3,919,278 and 3,919,279.
With the present invention, a small amount of non-volatile
by-products is formed in the reactor. However, such by-products can
be readily removed from the reactor together with the reaction
liquid when this and the catalyst dissolved therein are
continuously withdrawn from the reactor. The reaction liquid from
which the by-products have been removed is then pre-heated and
recycled to the reactor as inert solvent containing the catalyst.
Although the life of the catalyst is sufficiently long, the small
loss of catalyst during the prolonged operation necessitates an
occasional addition of fresh catalyst to the reactor or to the
recycling liquid.
When polyisocyanate is prepared, an isocyanatocarbamate is formed
as an intermediate product. As the boiling point of this
intermediate is higher than that of the polyisocyanate, the
preparation of the polyisocyanate is carried out by either a
process wherein the intermediate product is not distilled for the
reactor or by a process wherein the intermediate product is
distilled from the reactor together with the polyisocyanate and
alcohol, separated therefrom by distillation and recycled to the
reactor.
No special material of construction is required for the apparatus
for carrying out the process of the present invention. Commonly
used stainless steel or ordinary steel can be used with
impunity.
According to the process of this invention, an isocyanate fraction
of high purity (95% or higher) and an alcohol fraction can be
obtained in very high yields (about 95% or more) as pyrolytic
products of the carbamate at a very satisfactory reaction rate by
the effect of a very small amount of catalyst. Pure isocyanate may
of course be obtained by distillation of the isocyanate fraction
with any conventional method.
EXAMPLES
The present invention will now be illustrated in more detail by way
of examples. The following examples show by way of illustration but
not in a limitative sense the results of the pyrolysis of the
commercially most useful carbamate, diethyl
tolylene-2,4-dicarbamate [referred to hereinafter simply as
2,4-DCT(Et)] according to the two typical modes of reaction showing
the effects of the catalyst of this invention. A first mode is
directed to a refluxing method wherein the intermediate product is
refluxed to the reactor; the second mode is directed to a
non-refluxing method wherein the whole reaction product in the
vapor state is distillated from the reactor. Refluxing method: The
equipment included a 200 ml round-bottomed flask equipped with an
inlet for feeding the carbamate, a distillation tower of 25 mm I.D.
filled with MacMahon packings to a height of 100 mm. A stirrer or
alternatively a capillary for introduction of nitrogen into a
catalyst solution for promoting diffusion of the molten carbamate
into the catalyst solution or contact between them instead of
stirring the catalyst solution and a thermocouple for adjusting the
reaction temperature. After charging the flask with given amounts
of a catalyst and a solvent, a carbamate was molten in a vat for
fusing the starting material and supplied at a constant flow rate
through a needle valve into the flask maintained at a given
temperature and pressure. When the capillary was used, the flow
rate of the nitrogen was only that necessary to maintain good
contact between the catalyst solution and the molten carbamate. The
amount of nitrogen necessary for this purpose was only about 1
ml/minutes, contrary to the above mentioned U.S. Pat. Nos.
3,919,278 and 3,919,279, where the flow rate of nitrogen as carrier
is at least 100 ml/min. and actually at least 1 liter/min. Thus, it
is evident that the very small amount of nitrogen used in the
present invention does not function as a carrier. When a stirrer
was used, nitrogen was not at all used. The isocyanate as end
product and the alcohol produced by the pyrolytic reaction were
separated in the distillation tower from the intermediate product
(e.g., a monoisocyanatomonocarbamate) and from the high-boiling
by-products and were taken out in vapor form from the top of the
tower. The vapor product was first cooled with a water-cooled
condenser connected to the top of the tower to collect the
condensed isocyanate and then cooled in a dry ice trap where the
alcohol was condensed and collected. After a continuous supply of
carbamate for 1.5-3 hours under given reaction conditions, the
distillation rates of the isocyanate and the alcohol for the
supplied carbamate were calculated from the weights of the
isocyanate and the alcohol collected. The compositions of both
products were measured by NCO analysis, gas chromatography and high
speed liquid chromatography and thereafter the yields of NCO
groups, isocyanate and alcohol from the starting carbamate were
calculated. By weighing the contents of the flask after completion
of the pyrolytic reaction, the rate of formation of non-distilled
by-products having high boiling points as a non-distilled fraction
was calculated on the basis of the carbamate fed and then a
material balance was calculated. Prior to the pyrolytic reaction,
the reaction solvent was pre-heated to a temperature 50.degree. C
higher than the reaction temperature under a pressure lower than
the actual reaction pressure in order to remove any moisture and
other contaminants having low boiling points. Non-refluxing method:
This method contemplates the immediate removal from the reaction
system of all of the vaporized products formed during the pyrolytic
reaction. While in the refluxing method, yield and synthesis rate
of the desired isocyanate are influenced by the distillation tower
incidental to the reactor, in the non-refluxing method a comparison
is possible of results of the reaction under uniform conditions and
is thus suitable for judging the effect of the catalyst.
This method was carried out in the quite same manner as the fluxing
method except that the distillation tower was detached from the
reaction flask and a fractional condenser was directly attached to
the reaction flask.
EXAMPLE 1
According to the refluxing method, the flask was charged with 0.152
g of zinc naphthenate as catalyst and 50 g of a petroleum fraction
of paraffin series (NURAYN 165 produced by Exxon Corp.) as solvent.
To the catalyst solution was added, while maintaining the flask
under a reduced pressure of 20 mmHg and at 250.degree. C by heating
with a mantle heater, 2,4-DCT (Et) (in molten state and maintained
at 150.degree. C) at a rate of 0.39 g/min. for 91 minutes. The flow
rate of nitrogen introduced through the capillary was about 1
ml/min. corresponding in total to only 3 mol % of the supplied
carbamate. After a few minutes from the initiation of supply of the
carbamate, the products were distilled at a constant rate at
122.degree. C. The distilled product was subjected to fractional
condensation at 30.degree. C to collect the isocyanate fraction and
then passed through a dry ice trap where ethanol formed by the
pyrolytic reaction was condensed and collected. Using 35.4 g of
dicarbamate, 21.9 g (61.9% in the distillation rate) of isocyanate
fraction and 12.1 g (34.2% in the distillation rate) of ethanol
fraction were obtained. By analysis, the isocyanate fraction
contained 47.3 % by weight of NCO groups and was composed of 96% by
weight of desired tolylene-2,4-diisocyanate (referred to
hereinafter simply as 2,4-TDI) and 4.0% by weight of ethyl
tolylene-2,4-monoisocyanatomonocarbamate of the formula: ##STR3##
[referred to hereinafter as 2,4-ICT(Et)] as an intermediate
product, while the ethanol fraction contained 96% by weight of
ethanol. The production rate of non-distilled by-products was 2.3%
by weight including what remained in the tower. The recovered
products constituted 98.4% based on the starting 2,4-DCT(Et).
The yields of the products were calculated as follows:
Nco groups: 92%
2,4-TDI: 91%
2,4-ict (et): 3.1%
ethanol: 95%
A similar pyrolytic reaction was carried out using the recovered
reaction solution containing the catalyst from which by-products
had been removed. The results of this reaction were nearly the same
as above and no deterioration of the catalyst was observed. A
second and similar pyrolytic reaction was carried out using a 700
rpm stirrer instead of the nitrogen flow. Also in this case,
results of the reaction were nearly the same as above, thus proving
that no carrier gas is necessary.
REFERENCE EXAMPLE 1
For purpose of comparison, a pyrolytic reaction was carried out in
the same manner as described in Example 1 except that no catalyst
was used. As a result of the reaction, the distillation rates of
the isocyanate and ethanol fractions were 53.0% and 31.8%,
respectively, and the production rate of the non-distilled
by-products was 12.9%. The isocyanate fraction contained 45.2% of
NCO groups an was composed of 89% TDI and 11% ICT while the ethanol
fraction contained 98% ethanol. The yields were found to be:
Nco groups: 76%
Tdi: 72%
ict: 7.3%
ethanol: 90%
A comparison of the above results with that of Example 1 revealed
that although the pyrolysis of the starting carbamate took place
nearly to the same extent in both cases, the yields of NCO groups
and TDI in the case of Reference Example 1 were very low but the
production rate of non-distilled by-products was about 6 times
greater than in Example 1. A result of this comparison indicates
that in the pyrolytic reaction carried out in the absence of the
catalyst, the selectivity to TDI is considerably decreased. Thus, a
comparison between Example 1 and Reference Example 1 teaches that
the catalyst has a remarkable effect for selectively promoting
pyrolytic conversion of carbamate groups into NCO groups and
inhibiting occurrence of side reactions.
EXAMPLES 2-4 AND REFERENCE EXAMPLE 2
For the purpose of illustrating the effect of the catalyst more
explicitly, the pyrolytic reaction of carbamates was carried out in
the presence and in the absence of catalyst according to the
non-refluxing method. The reactor was charged with 0.10 g of
catalyst and 50 g of a petroleum fraction of the naphthene series
(JWS 8510B, Exxon Corp.) as solvent. To the catalyst solution was
added, while maintaining the reactor under a reduced pressure of 20
mmHg at 250.degree. C, 2,4-DCT(Et) in molten state and maintained
at 150.degree. C at a rate of 0.5 g/min. for about 2 hours and the
pyrolytic reaction was carried out continuously. A result of the
experiments is shown in Table 1.
Table 1
__________________________________________________________________________
Pyrolysis of diethyl tolylene-2,4-dicarbamate according to the
non-refluxing method Distillation rate (%) Composition of TDI
Ethanol TDI fraction (%) Yield (%) Synthesis rate of Example frac-
frac- NCO NCO TDI No. Catalyst tion tion groups TDI ICT groups TDI
ICT Ethanol** g/liter/hr
__________________________________________________________________________
2 Aluminum sec-butylate 64.8 29.0 35.9 63 29 74 62 23 83 220 3 Zinc
N,N-ethyl- phenyl-dithio- 69.5 28.9 35.1 61 30 77 65 25 81 230
carbamate 4 Zinc dodecyl- 65.3 26.7 36.3 62 34 75 62 27 74 220
benzene-sulfonate Ref 2* None 66.7 21.5 23.2 21 68 49 22 55 55 75
__________________________________________________________________________
*Reference Example 2 **The yield of ethanol was calculated from the
amount of ethanol containe in the ethanol fraction.
Almost similar results were obtained when a 700 rpm stirrer was
used instead of the small nitrogen flow.
When the catalyst was used, the TDI content in the TDI fraction,
the yield of TDI and the synthesis rate of TDI in each Example were
about 3 times as much as those in Reference Example 2 wherein no
catalyst was employed, thus indicating the remarkable technical
effect of the catalyst, especially for promoting selectivity in the
reaction.
EXAMPLES 5-18
Using various kinds of catalyst, the pyrolytic reaction was carried
out in the same manner as described in Example 1 in which the
refluxing method was employed, except that the amount of catalyst
used was 0.1 g, the feed rate of diethyl tolylene-2,4-dicarbamate
was 0.4-0.5 g/min., the rotational speed of the stirrer was about
700 rpm and the reaction time was 1.5-3 hours. The catalyst was
used so that the concentration of the metal contained therein was
within the range of 0.01-0.1% by weight based on the solvent used.
The results of the experiments are shown in Table 2 below.
Table 2
__________________________________________________________________________
Catalytic pyrolysis of diethyl tolylene -2,4-dicarbamate according
to the refluxing method Distillation Composition of TDI rate (%)
Production rate fraction (%) Yield (%) Ex. TDI Ethanol of
non-distilled NCO NCO No. Catalyst Fraction Fraction material (%)
Group TDI ICT Group TDI ICT Ethanol*
__________________________________________________________________________
5 Aluminum sec-butylate 63.3 34.0 2.9 46.6 95 4.2 94 92 3 97 6 Zinc
N,N-ethylphenyl- dithiocarbamate 65.4 34.2 0.4 46.8 95 5.0 97 95 4
98 7 Zinc acetate 63.9 34.0 0.3 47.2 96 3.7 95 94 3 95 8 Zinc
stearate 60.9 32.6 3.5 46.8 95 5.4 90 88 4 92 9 Zinc benzoate 63.5
33.4 1.3 46.9 93 6.5 94 91 5 97 10 Zinc p-methyldithio- cathecolate
62.7 33.5 0.8 46.3 95 3.2 92 91 2 96 11 Zinc acetylacetonate 65.2
33.4 0.2 44.1 88 8.4 91 88 7 94 12 Tin acetate 60.0 31.6 3.9 46.8
95 4.7 89 87 3 90 13 Copper naphthenate 62.4 33.2 5.5 47.1 96 2.7
93 92 2 93 14 Titanium oxalate 64.1 31.3 3.2 44.5 90 6.4 91 88 5 90
15 Vanadium acetyl- acetonate 63.5 33.4 3.0 46.7 96 1.8 94 93 1 95
16 Iron naphthenate 62.4 32.9 1.1 46.0 92 8.0 91 88 6 90 17 Cobalt
naphthenate 63.2 33.4 2.7 46.4 96 0.8 93 93 1 94 18 Nickel
naphthenate 60.7 33.1 3.1 46.2 94 4.6 89 87 3 92
__________________________________________________________________________
*The yield of ethanol was calculated from the amount of ethanol
contained in the ethanol fraction.
The results tabulated in Table 2 show that TDI having a purity of
at least 95% was obtained in high yields (nearly 90-95%). A test
piece of SUS 27 steel was placed in the reaction flask during the
reaction but no corrosion was observed thereon after the 20-hour
test.
Using a tower of 30 mm I.D. filled with MacMahon packings to a
height of 300 mm, the TDI fraction thus obtained containing 95% TDI
and 5% ICT was continuously distilled whereby a pure TDI fraction
having a purity of 99.8% was obtained at a distillation rate of
98%.
EXAMPLE 19
As a result of the pyrolytic reaction carried out in the same
manner as described in Example 1 except that 0.1 g of zinc
hydroxide and 0.35 g of naththenic acid were used as catalyst, the
distillation rates of TDI and ethanol fractions were 61.5% and
34.4%, respectively, and the TDI fraction was composed of 46.6% NCO
groups, 96% TDI and 3.0% of ICT. The yield of NCO groups was 91%,
that of TDI 90%, ICT 2% and ethanol 99%.
EXAMPLE 20
A pyrolytic reaction was carried out in the same manner as
described in Example 1 except that a mixture of diethyl
tolylene-2,4-dicarbamate and diethyl tolylene 2,6-dicarbamate in a
mixing ratio of 80:20 parts by weight was used as starting material
in place of diethyl tolylene-2,4-dicarbamate alone. As a result of
this pyrolytic reaction, the distillation rates of TDI and ethanol
fractions were 63.0% and 33.4%, respectively, and the TDI fraction
was composed of 46.0% NCO groups, 95% TDI and 2.3% ICT. The yield
of NCO groups was 92%, that of TDI 91%, ICT 2% and ethanol 97%.
It is understood that the preceding representative examples may be
varied within the scope of the present specification, both as to
reactants and reaction conditions, by one skilled in the art to
achieve essentially the same results.
As many apparently widely different embodiments of this invention
may be made without departing from the spirit and scope thereof, it
is to be understood that this invention is not limited to the
specific embodiments thereof except as defined in the appended
claims.
* * * * *